An $11 million campaign contribution from an Arizona nonprofit that gave money to a group fighting Gov. Jerry Brown's tax initiative represents the largest case of campaign "money laundering" in state history, California's political watchdog agency charged Monday.

"You are required to disclose the true source of a donation," said Gary Winuk, the chief of enforcement for the Fair Political Practices Commission. "So, if you run your contribution through another committee, you're required to say that you ran it through that committee. If you don't, it's money laundering."

Americans for Responsible Leadership reported the source of its contribution to a political action committee that is active in the November election after the state Supreme Court ruled unanimously Sunday that the group must turn over its records.

The disclosure indicates ARL received the money from a group called Americans for Job Security through a second intermediary, the Center to Protect Patient Rights, the California Fair Political Practices Commission said Monday.

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Both are federally registered nonprofits that are not legally required to disclose the source of their funds.

"Today, Americans for Responsible Leadership said they failed to disclose themselves as an intermediary for the true contributor, so they have committed campaign money laundering," Winuk said.

Both groups have been active in the presidential race, pouring tens of millions of dollars into swing states for independent expenditure ads supporting GOP presidential nominee Mitt Romney and other Republican candidates.

The Arizona group did not initially disclose that it received money from those two groups, although it filed an amended state report Monday.

Attorneys for Americans for Responsible Leadership sent a letter to FPPC regulators that stated, "neither ARL nor CPPR admit any wrongdoing."

But California Common Cause, a campaign finance watchdog group that first initiated a complaint with the FPPC, was elated by the disclosure of donors.

"This is money laundering, and these folks are trying to hide their identities from the voters," Phillip Ung told KCRA 3.

Ung said the nonprofit groups were fronts for Charles and David Koch of Koch Industries, based in Kansas.

Each brother is worth $25 billion, according to Forbes Magazine, and are 12th on the list of the world's biggest billionaires, according to Forbes.

California's campaign finance watchdog enforces the state's campaign finance and disclosure laws, and can issue fines and penalties for noncompliance, although it refers legal violations to law enforcement agencies. The board's chairwoman, Ann Ravel, was appointed to the post by Brown.

Attorney General Kamala Harris, a Democrat, and the state's campaign finance watchdog had been seeking the donation records to determine whether Americans for Responsible Leadership is complying with state campaign disclosure laws.

Her office had no immediate comment on Monday's disclosure, which came on the eve of Election Day.

Federal law allows nonprofits to keep the identities of their donors confidential. But California regulations require donors to be identified if they give to nonprofits with the intention of spending money on state campaigns here.

ARL's $11 million donation went to the Small Business Action Committee PAC, which opposes Brown's tax initiative and a separate initiative that would limit unions' ability to collect money for political causes.